1. Field of the Invention
The present invention generally relates to electroplating or electrodeposition of metals and more particularly to an improved aqueous non-cyanide electroplating solution useful as a strike bath for deposition of copper on zinc or other metals.
2. Prior Art
The electrodeposition of metals from aqueous solutions is well-known. Typically, an electroplating system includes an electroplating bath and two or more electrodes immersed in the bath. The cathode is the object to be plated. The anode may be either carbon or a piece of the metal to be plated on the cathode. When the anode is the plating metal, it is consumed during the electroplating process, providing a constant source of metal ions to the plating bath.
In electroplating metals such as copper on active metals such as zinc, it is necessary to first make the initial deposit from a strike bath in order to avoid immersion plating, that is, chemical deposition of metal. The strike bath ordinarily contains a sufficiently low concentration of metal ions so as to avoid the undesired chemical deposition of immersion plating.
Copper-containing strike baths usually utilize copper as a cyanide. Cyanides of other metals are also used in strike baths. However, cyanides are extremely dangerous to use. If the pH of the strike bath drops to 7.0 or becomes acidic (i.e., below pH 7.0), then there is real danger of producing hydrogen cyanide gas in the process. Hydrogen cyanide gas is colorless, odorless and deadly. Toxic metal cyanides are also hazardous. If the electroplater happens to absorb the dissolved cyanide, as through a skin cut or the like, injury or death can also occur. Moreover, the used cyanide-containing bath is difficult to dispose of safely without harming the environment. In addition, if an attempt is made to recover the metal cyanide from the used strike bath, special equipment and techniques must be employed which raise the overall cost of the electroplating operation.
Various attempts have been made in the past to carry out electroplating steps without the use of cyanides. See, for example, U.S. Pat. No. 3,475,293 which calls for the use of diphosphonates or monoamino lower alkylene phosphonates in electroplating divalent metal ions. See also U.S. Pat. Nos. 3,706,634 and 3,706,635 which utilize various types of phosphonic acids as complexing agents. Unfortunately, immersion plating may still occur. Metal deposited by immersion plating is spongy and has poor adhesion so that subsequent metal deposited electrically tends to crack and peel.
Newer cyanide-free solutions for use as strike baths are exemplified by U.S. Pat. No. 3,879,270 which utilizes cyanuric acid or a salt thereof, U.S. Pat. No. 3,914,162 which employs a carboxy alkylene amino di(methylene phosphonic acid) and U.S. Pat. No. 3,928,147 which employs an organophosphorus chelating agent. Such solutions work with indifferent results.
A problem not addressed by the prior art but which needs to be solved for best plating results is the unwanted contamination by iron which occurs during the electroplating of copper on zinc. The iron is typically derived from the parts which are to be plated. Once the iron begins to deposit during electroplating (which begins to occur noticeably after a few days of electroplating), it forms a metal complex with the desired metal being deposited, which complex weakens the adhesion between the metal being plated and the plating metal, specifically the adhesion between the copper being plated and the zinc being plated by the copper.
Accordingly, it would be highly desirable to be able to provide an improved electroplating bath, more particularly, a strike bath useful for the plating of copper on zinc or the like without interference from iron contamination and without the necessity of using harmful dangerous chemicals such as cyanides in the bath. Such bath should be inexpensive, efficient and easy to make up and dispose of without contamination of the environment and without risk to the plater.